This invention relates to force-measuring apparatus such as a weighing scale and more particularly to force-measuring apparatus compensated for the position of the load on the apparatus.
It is well known that differences in load position on weighing scales reduces the accuracy of weighing measurements and must be more or less compensated in accordance with the required accuracy of the weighing apparatus. In moment-insensitive single load cell scales where the weight-receiving platter and the base support are attached directly to the load cell the effect of load position differences is reduced by the design of the load cell itself. Such a load cell typically includes a load responsive spring element, or counterforce, and strain gages mounted on the counterforce and connected together in an electrical bridge circuit. The load cell is usually designed so that (1) a change in resistance of one strain gage caused by a change in the position of the load is accompanied by a change in the resistance of the other gages such that the output of the bridge circuit tends to remain unchanged and/or (2) the strain gages are positioned in areas or in directions that cause them to be relatively insensitive to changes in the position of the load. A dual beam moment insensitive load cell with the strain gages centered on and parallel to the central axis of the beams employs both these techniques.
Further reduction of load position errors is usually required and has been accomplished in several different ways. One technique involves honing of the counterforce to slightly alter the geometry of the load cell. This technique produces improvements but is very laborious and time consuming.
Another technique, suggested in U.S. Pat. No. 3,576,128 to Lockery, involves the use of compensating resistors connected to the strain gages. A similar but considerably improved technique was suggested in U.S. Pat. Nos. 4,380,175 and 4,453,609 Griffen and Griffen et al, respectively. Substantially improved results were obtained with a significant reduction in labor by positioning of the strain gages in certain ways combined with the use of compensating resistors in circuit with certain of the strain gages.
In U.S. Pat. No. 4,482,022 Komoto, a force-measuring transducer is calibrated for a center loading position and auxilary transducers are provided for detecting displacement of the load from the center calibrated position. The outputs of the auxilary transducers are employed to adjust the gain of the force measuring transducer to correct for any detected displacement from the calibrated position.